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DEPARTMENT OF COMMERCE 

BUREAU OF STANDARDS 
S. W. STRATTON, Director 



TECHNOLOGIC PAPERS OF THE BUREAU OF STANDARDS NO. 135 

[Issued October 16, 1919] 



BEHAVIOR OF WROUGHT MANGANESE BRONZE 
EXPOSED TO CORROSION WHILE UNDER TENSILE 
STRESS 



By P. D. Merica, Physicist, and R. W. Woodward, Associate Physicist 



^.27bl>S" J 



INTRODUCTION 



In the course of a previous investigation by the authors ^ of a 
series of failures by fracturing of brass and bronze bolts it was 
suspected that many cases of such failure were due not to defec- 
tive material but to the fact that the bolts had been overstressed 
in service. Careful examination of these bolts revealed a normal 
and sound structure and satisfactory mechanical properties; the 
bolts were comparatively free from initial stresses which would 
have caused corrosion cracking. No other cause of failure could 
be well assigned other than that they had actually been over- 
stressed in tightening up. 

The question was thus raised, What service stresses may with 
safety be applied to physically normal wrought brass and bronze 
of the types ordinarily used for structural purposes? The failed 
bolts in question had probably not been stressed above from 
1 5 ooo to 20 coo poimds per square inch ; their tensile strength 
was about 6o ooo, their yield point about 30 000 poimds per 
square inch. The service stresses were thus presumably not above 
the yield point. 

'p. D. Merica and R. W. Woodward, Failure of Brass, Technologic Paper No. 82 of the Bureau of 
Standards; 1917. 

120129°— 19 , , 



Mono^'api! 






2 Technologic Papers of the Bureau of Standards 

Jonson ,2 in a most interesting investigation, showed that 
brass rods exposed to the action of concentrated ammonium 
hydroxide and subjected at the same time to the very gradual 
application of tensile stress would break with Httle elongation at 
any values of the stress greater than the yield point of the ma- 
terial. Fracttue occurred within from 6 to 20 days. 

The effect of combined tensile stress and surface corrosion is 
to diminish, apparently, the stress at which fracture will occur. 

The corrosive action of this solution is very severe, and yet 
many, including the authors, believed that its action under these 
conditions was not different in principle, but only more rapid, 
than that of water or moist air in which brass or bronze bolts 
might actually be placed in service. It was believed that Jon- 
son's work demonstrated that wrought brass or bronze could be 
used only for very low stresses and with high factors of safety 
whenever corrosion was to be expected. In view of the fact 
that these materials would supplant steel usually only because 
of their superior resistance to corrosion, the verdict of the work 
seemed to Hmit very largely the usefulness of brass and bronze, 
for structural ptuposes at least, until more reassuring information 
concerning their behavior in service might be obtained. 

The authors have, therefore, imdertaken to discover whether 
corrosion by water and moist air will produce the same effect in 
combination with the simultaneous application of tensile stress 
as that by ammonium hydroxide. Bars of wrought manganese 
bronze were exposed to corrosion in water and moist air while 
at the same time under tensile stresses of different values. Al- 
though the period of exposure has been to date only two years, 
it is thought that a report of the behavior of these bars during 
that relatively short period will be of interest to those using and 
manufacturing this material. 

Since much longer periods than the few days of Jonson's tests 
were contemplated, small test frames were used for these tests 
rather than standard tensile-testing machines for the application 
of tensile stress to the specimens. Twelve such frames were made, 
in each of which a brass-bar specimen could be placed in tension 
of any required amount and the value of the stress measured by 
strain gage measttrements on a steel reference bar placed "in 
series" with the brass-test specimen; that is, such that the same 
total load was borne by the steel reference bar as by the brass-test 
bar. 

' E. Jonson, The Fatigue of Copper Alloys, Proc. Am. Soc. for Testing Materials, 40, p. loi; 1915. 

OCT •••23 \m 



/f-2?S'3e 



Corrosion-Stress Tests of Manganese Bronze 3 

A sketch of the frame used is shown in Fig. i . 

Each of the frames consists of two blocks of steel, a a, which can 
be braced apart by the two rods b b, of which one end is secin-ed 
by pins d d against rotation, while the others fit in the bored 
holes, e e. The brass specimens, as well as the steel reference 
bars, are threaded at the ends and secin"ed by nuts, c c, at the 
ends and by the coupling nut i at the center. A ball and socket 
joint is provided at// to secure axial loading. The center mem- 
ber of these frames consisting of the steel reference bar and the 
brass test bar can be placed under a tensile load of any desired 
value by the adjustment of the nuts g g against the block a. 

The test specimens were cut from ^ inch round bars ; they were 
ii>^ inches long and 0.50 inch in diameter and had standard 
^-inch threaded ends. The steel reference bars for the exten- 
someter measurements were also 11% inches long, were of two 




1 2 — rt 



Fig, I. — Frame used in the stress-corrosion tests 



diameters, 0.420 and 0.350 inch, and had also standard ^-inch 
threaded ends. 

An alloy-steel rod kindly furnished by the Midvale Steel Co. 
was used for the preparation of the steel reference bars. This 
was heat treated to produce a high elastic Umit, and the section 
of the rod was so chosen as to allow of the utilization of the full 
elasticity of the rod in each individual measmrement of stress. 

II. MATERIALS USED FOR TEST 

In this first series of tests only one type of brass was used; 
that is, manganese bronze. The American Brass Co. very kindly 
furnished ^-inch round rods of this material in three tempers or 
degrees of hardness, Nos. M237, M238, and M239, as well as the 
results of chemical analyses and of mechanical tests of the rods. 
These data are given in Table i. 



4 Technologic Papers of the Bureau of Standards 

TABLE 1. — Chemical Analysis and Mechanical Properties of the Three Samples of 
Wrought Manganese Bronze Used in the Tests" 



Chemical composition: 

Copper per cent 

Zinc * do. . 

Tin do.. 

Lead do.. 

Iron do.. 

Manganese do. . 

Mechanical properties: 

Tensile strength pounds per square Inch 

Yield point do.. 

Elongation in 2 inches per cent 

Reduction of area do. . 

Brinell hardness « 

Scleroscope hardness <: 



Rods No. 
M237 


Rods No. 
M238 


56.64 


56.87 


40.94 


41.07 


1,26 


1.20 


.12 


.08 


.85 


.68 


.19 


.10 


84 700 


89 050 


44 050 


59 550 


29 


22 


25 


19 


117 


137 


36 


43 



Rods No. 
M239 



56.60 

41.01 

1.18 

.11 

.91 

.19 

101 650 
73 350 
14 
10 
158 
49 



o These values were furnished by the American Brass Co. 

6 .By difierence. 

c Taken at the center of the section of the rod. 

These rods as furnished were in a condition of more or less 
initial stress, and, as it was not desired to have the interpretation 
of the results of these tests complicated by their presence, the rods 
were annealed between 285 and 315° C in a salt bath for five 
hours and slowly cooled. This temperature, it has been shown, ^ 
is sufficient to relieve initial stress in manganese bronze, but does 
not markedly soften the material. 

Determination of the proportional limit of a bar of each group 
after this annealing showed the following results: 



Manganese bronze rods 


Proportional 
limit 


No. M237 


Lbs./in.s 
25 000 


No. M238 


35 000 


No. M239 


42 500 







The modulus of elasticity of all of the bars was appreciably the 
same; that is, 16 000 000 pounds per square inch. 

III. PROCEDURE OF TEST 

From each group of brass rods of the same temper were pre- 
pared four test specimens. These were placed in the frames and 
tensile stresses of different values, given in Table 2, applied to 



' Ix)c. cit. 



Corrosion-Stress Tests of Manganese Bronze 5 

them. Nine test specimens — ^that is, three of each group — were 
exposed to corrosion. Of each group one specimen was stressed 
just to the proportional Umit, one was stressed to a value of the 
stress just above the proportional limit, and one to a value just 
below it. Three test specimens, one of each group, were stressed 
just to the proportional limit, and then covered with paraffin 
as a protection against corrosion; these specimens were preserved 
as comparison specimens in the laboratory during the period of 
test. 

TABLE 2. — Proportional Limits of the Test Specimens and Approximate Test Stresses 

Applied to Them 



Frame 


Specimen 


Proportional 
limit 


Approximate 
test stress 


1 


M237-A 
M237-C 
M237-D 
M238-A 
M238-C 
M238-D 
M239-A 
M239-B 
M239-C 
M237-B 
M238-B 
M239-D 


Lbs./in.2 
25 000 
25 000 
25 000 
35 000 
35 000 
35 000 
42 500 
42 500 
42 500 
25 000 
35 000 
42 500 


Lbs./in.2 
15 000 


2 


25 000 


3 


30 000 


4 


20 000 


5 


35 000 


6 


40 000 


7 


25 000 


8 


42 500 


9 


47 500 


10 


30 000 


11 


40 000 


12 ^ 


45 000 







For the measurement of stress a 5 -inch strain gage reading to 
0.000 1 inch was used, and on each bar two sets of holes for the 
gage points were placed on opposite sides. The difference in the 
strain-gage reading before and after loading the bars by tightening 
up the nuts g g, ox unloading by loosening the nuts, allowed of the 
calculation of the applied stress. That the stresses were applied 
approximately axially is shown by the fact that the extensions of 
the two opposite fibers of each bar usually agreed in value within 
less than 5 per cent ; in a few cases the agreement was only within 
10 per cent. , 

The average value of the extensions on the two opposite sides 
was used in the calculation of stress. 

After applying the desired stresses to all of the specimens in the 
frames, they were allowed to lie for a few days and were then re- 
gaged. Only insignificant changes in length and consequently of 
stress had occurred in that time. The frames were then wrapped in 
cloth and painted with red lead and the steel reference bars covered 



6 Technologic Papers of the Bureau of Standards 

with a layer of paraffin as a protection against corrosion. All 
gage-point holes were also carefully wrapped with cloth and 
paraffined. The portion of the brass test specimens in frames i 
to 9 between gage-point holes was left bare and unprotected; the 
test specimens in the frames lo to 12 were paraffined. 

IV. EXPOSURE OF TEST FRAMES 

The frames i to 9, inclusive, were then placed near the top of 
a water surge tank in the engine room, such that they were al- 
ternately immersed in water and exposed to the air. Frames 10, 
1 1 , and 1 2 were laid away in the laboratory. This was done on 
December 22, 191 6. On March 2, 191 7, the frames were removed 
and examined. No cracks or fractures had appeared; the brass 
had tarnished badly. On March 13 the tensile stress on each 
test specimen was relieved and measured; on March 15 the test 
stress was again applied, measured, and on April 6 the frames 
were put back in the tank. On June 1 2 they were again removed 
and examined. No cracks had appeared, but the corrosion had 
progressed, and the stu'faces of the bronze bars were of a green- 
ish-brown color. The frames were placed by error at this time 
outside the tank and were exposed to the fumes and smoke of 
the boiler room, but they remained fairly dry. On October 10, 

191 7, the frames were replaced in the tank. On March 4, 1918, 
the frames were removed for examination. The specimen No. 
M239-B in frame 8 had fractured, evidently quite recently, judg- 
ing by the brightness of the fractured svirfaces. Fig. 2 shows 
the appearance of this frame and specimen after fracture. The 
other specimens were qviite sound. All of the frames except 
No. 8 were replaced in the tank on March 10. On December 30, 

1918, upon searching for these frames, it was discovered that 
about August i they had been removed from the tank by the 
plumbers and placed back of it. When found, frames Nos. 2, 6, 
and 9 were entirely immersed in dirty water; Nos. 3, 4, and 5 
were partially immersed, lying on top of the others. Nos. i and 

7 were in another location, which wks dry at the time of exami- 
nation. The latter frames were quite dirty, as were, indeed, all 
of the frames, being covered with black mud, and they had evi- 
dently been in the water or had been sprayed with water from 
the hose while in that location. 

AH of the brass specimens after washing ofif the mud showed 
that they had been moderately corroded, the stufaces being 
covered with a heavy, dark reddish-brown patina. 



Bureau of Standards Technologic Paper No. 135 





Fig. 2. — Photograph of frame No. 8 with 
specimen No. M 2jg-B , fractured after 
about one year's exposure to corrosion 
under a tensile stress of approximately 
4 J 000 pounds per square inch 



Fig. 3- — Photograph of frame No. g with 
specimen No. M 2jg-C after two years' 
exposure to corrosion under a tensile 
stress of about 4S 000 pounds per square 
inch 



Corrosion-Stress Tests of Manganese Bronze 



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8 Technologic Papers of the Bureau of Standards 

Specimen No. 238-D, in frame 6, was fractm^ed. No other 
fractures or cracks were noticed in any of the specimens. Photo- 
graph, Fig. 3 shows the appearance of frame No. 9 as it was found 
on December 30. 

The stresses in some of the frames were found to have been 
sHghtly reduced during the last exposure period. It is suspected 
that upon removing the frames from the tank they were dropped 
or otherwise roughly handled, thus occasioning the loosening of 
the nuts. 

In Table 3 will be found the values of the stress applied to the 
different brass specimens as measured at each loading and un- 
loading. The first value in each case is the value measured on 
the steel bar. In the third column is the value of the extension 
produced in the brass bar, and in the second column of each 
group the value of the stress in the brass bar, calculated from 
its o^\^a extension (using 16 000 000 pounds per square inch as 
the modulus) . 

It will be noticed that the values of the stress calculated from 
the extensions of the steel bar agree well with those calculated 
from the extensions of the brass bar, except on the first loading. 
The brass specimens in frames 2, 3, 5, 6, 10, and 11 evidently 
5H[elded slightly and took a permanent set upon their first loading; 
the stress values of the second column have in this case, therefore, 
no meaning. 

V. CONCLUSIONS 

In considering the results of these tests it must be emphasized 
that the period of exposure — two years — during which the bars 
have been obser\'ed is relatively short in comparison with the 
periods for which such materials may actually be used in servnce. 
The test bars used were given a low temperatiu-e anneal, also, in 
order to relieve the initial stresses, and consequently their be- 
havior may differ during test from that of bars in which these 
stresses still remain. It is quite apparent, therefore, that any con- 
clusions to be derived from these results must be regarded as 
quite restricted in their definite application and as more or less 
tentative in their more general aspects. 

Within the period of exposure of two years no specimen of 
the wrought manganese bronze fractured tmder a stress below its 
proportional limit, and fom* specimens — Nos. M237-C, M237-D, 
M238-C, and M239-C — did not fracture nor crack luider a stress 
which caused sHght yielding and permanent set when first applied. 



Corrosion-Stress Tests of Manganese Bronze 9 

Fracture or cracking did not occur in any bars stressed to values 
below 35 000 pounds per square inch. This value does not repre- 
sent the highest value of the tensile stress withstood during the 
period of test; specimen M239-C in frame 9 was still quite soimd 
at the end of two years while under a stress of from 45 000 to 
48 000 poiinds per square inch. Its behavior is considered some- 
what anomalous, however, since specimen M239-B of the same 
temper fractured within about 18 months under a stress of only 
41 000 to 45 000 pounds per square inch. It is predicted that the 
former specimen will eventually fracture under the higher stress. 

One specimen with a proportional limit of 35 000 pounds per 
square inch fractured under a tensile stress of approximately 
40 000 pounds per square inch, and another with a proportional 
limit of 42 500 povmds per square inch under a stress of approxi- 
mately 43 000 pounds per square inch. 

None of the bars in frames 10, 11, and 12, which were protected 
against corrosion, were fracttured at the end of the two years. 

The results of these tests seem to be partially, at least, in con- 
formity with the conclusions reached by Jonson from his work 
using ammonium hydroxide as a corroding medium. His con- 
clusion was that brass or bronze might not be subjected to cor- 
rosion (in ammonium hydroxide) while under a tensile stress 
greater than 20 000 pounds per square inch or greater than 
5000 pounds per square inch above the yield point, without 
danger of failure. The authors' tests indicate that the propor- 
tional limit is to be regarded as the maximum safe stress for 
bronze of harder tempers, but that it is not certain this limit may 
not be slightly exceeded in materials which are soft; that is, free 
from work hardness. 

Only further expostu-e tests, which are now proceeding, will 
decide this point. It is from the practical standpoint very de- 
sirable to know whether it is permissible to tighten a bolt of 
brass or bronze until it yields slightly or whether rigid care must 
be exercised that the load applied in tightening is at no time 
above the yield point. 

Washington, February 27, 191 9. 



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